Initially photographed and considered to be a Caspian Gull this individual First-winter bird in South Essex at the end of 2011 has bugged me a little as it just didn't seem to fit a typical Caspian Gull, indeed it appears to be closer to Heuglin's Gull. I kept returning to the images taken of this bird and after some research I have obtained some fairly positive comments from very prominent Gullers with more experience with heuglini than I on the bird with the caveat that as most heuglini should perhaps be closer to juvenile in plumage for the time of year. However had this bird been photographed in January-March and the Middle East then there would probably be no problem with it being a heuglini.
However I have located several images of probably or presumed Heuglin's Gull in December, such as on the Gull Research Org website linked here:
That show a bird extremely similar to my bird here, encouraging me to continue to seek further comments to try and determine whether it is a Heuglin's or just an Casp.
I would welcome comments on this bird to further its identification as to either heuglini, cachinnans or other.
Heuglin's Gull (putative) - Essex, 10th December 2011
The white head is a stand out feature with heavy hind-neck streaking creating a bold 'shawl'. The Upper-parts are well marked and bluer than on Caspian Gull, and this can be compare to the nearby Herring Gull. The Scapulars have pale tips and fringes, often bold dark shaft streaks and transverse bars.
The bills is already becoming pale and not as expected on a Casp.
Comments largely as above but here note the single replaced Lesser Covert, the Tertial pattern and the overall general wear and abrading to the feathers.
It has a more Heuglini feel to it than a Caspian.
This raised wing image shows a small, if any, inner primary window. On most, though not all, Caspian Gulls there is a small window, not as pronounced as on Herring Gull.
The underwing is overall palish with darker flecking to the wing coverts.
Again the head is strikingly white, offset by the heavy hind-neck streaking. Here the head looks smallish with flattened crown whilst the eye remains rather large looking.
Just visible it the tail pattern; narrow black band with obvious broad dark bars more reminiscent of heuglini perhaps rather than Caspian.
Prominent bulging upper-breast.
Here the bird is in aggressive mode which gives the opportunity to study the open wing more closely.
The inner primaries show paler inner primaries, but not extensively so and barely constitutes as a pale 'window'.
Note also the hind neck streaking and how obviously it is demarcated from the head and white neck sides.